What are some tips for becoming a better copywriter?

Copywriting is an extraordinarily broad area and industry associations wouldn't neccessarilly help.

I currently run a writing consultancy in China and the only real tips that have made a difference for me are:

1. Write every day - whether you want to or not
2. Use article syndication platforms like e-zine articles to get exposure - you need to achieve diamond level status to prove your quality of work with those guys
3. Write in subject areas you have expertise in - unless you're paid to research other areas
4. Write some more - squeezing out more copy is always a good thing
5. Have a website and blog of your own - demonstrate your skills there and keep them updated (use live feeds from syndication sites to save effort)
6. Keep prices at a sensible level - only give your work away to promote business (article syndication), always charge a decent rate to clients - if you think a $1 an article is fair, you'll starve
7. Keep going - the first year is a nightmare, but after that clients will start coming to you and you'll be able to relax a little
8. Look for trade press to market to - don't spend too much time looking for big online (no money) or big media (low publication chance)

Good luck - it's a great way to earn a living, but it's a hard slog in the first instance.